In its simplest form, an aircraft airspeed indicator is a needle that moves relative to a graduated face roughly in proportion to incident air pressure sensed with a pitot tube. The pitot tube, the L-shaped probe often located on the fuselage near the cockpit, contains a static port and a pitot/dynamic pressure port. These two pressures sensed at those ports are applied to an aneroid mechanism that expands and contracts in relation to the dynamic pressure changes with changed airspeed, moving the needle. Reliable as the simple mechanical airspeed may be, redundancy is desirable. Redundancy can be achieved by a second mechanical altimeter, but a second altimeter would take up more dash board space and confuse the pilot's instrument scan.